This invention relates in general to drilling a wellbore and, in particular, to drilling an intersecting wellbore through a drill string including well casing or liner and a downhole drilling apparatus interconnected therein.
Without limiting the scope of the invention, its background is described in connection with drilling a wellbore for hydrocarbon production, as an example.
Heretofore, in this field, a typical drilling operation has involved attaching a drill bit on the lower end of a drill string and rotating the drill bit along with the drill string to create a wellbore through which subsurface formation fluids may be produced. As the drill bit penetrates the various earth strata to form the wellbore, additional joints of drill pipe are coupled to the drill string. During drilling, drilling fluid is circulated through the drill string and the drill bit to force cuttings out of the wellbore to the surface, and to cool the drill bit.
Periodically as the drilling of the wellbore progresses, the drill bit and drill string are removed from the wellbore and tubular steel casing is inserted into the wellbore to prevent the wall of the wellbore from caving in during subsequent drilling. Typically, after casing is inserted into the wellbore, the annulus between the casing and wellbore is filled with a cement slurry that hardens to support the casing in the wellbore. Thereafter, deeper sections of wellbore with progressively smaller diameters than the previously installed casing may be drilled.
Once a predetermined depth is reached for each subsequent section of wellbore, the drill bit and drill string are again removed from the wellbore and that section of the wellbore may be cased. Alternatively, however, a liner may be used to case an open section of wellbore instead of a full casing string. The liner, which is a string of connected lengths of tubular steel pipe joints, is lowered through the casing and into the open wellbore. At its upper end, the liner is attached to a setting tool and liner hanger. The liner hanger attaches the liner to the previous casing such that the casing will support the weight of the liner.
The length of the liner is predetermined such that its lower end will be proximate the bottom of the open wellbore, with its upper end, including the liner hanger, overlapping the lower end of the casing above. As with the casing, after the liner is inserted into the wellbore, the annulus between the liner and the wellbore may be filled with a cement slurry that hardens to support the liner in the wellbore.
It has been found, however, that in many well drilling operations it is desirable to minimize rig time by utilizing the casing or liner string as the drill string for rotating a drill bit, which may be left in the wellbore upon the completion of drilling a section of the wellbore. As such, this procedure does not require the use of a separate liner or casing upon the withdrawal of the drill bit and drill string as in conventional drilling operations, and thereby reduces the time needed to drill, case and cement a section of wellbore.
For example, attempts have been made to utilize the casing or liner string as the drill string along with a drill bit that is rotatable relative to the casing or liner string. The drill bit is rotated by a downhole drill motor that is driven by drilling fluid. Upon completion of drilling operations, the motor and the retrievable portions of the drill bit may be removed from the wellbore so that further wellbore operations, such as cementing, may be carried out and further wellbore extending or drilling operations may be conducted. This system, however, requires the use of expensive and sometimes unreliable downhole drill motors and a specially designed drill bit.
Alternatively, other attempts have been made to utilize the casing or liner string as the drill string using conventional rotary techniques wherein the drill bit is rotated by rotating the entire casing or liner string. This approach, however, requires the use of a drill bit with minimal cutting structure, since a drill out could not be performed through a typical drill bit having a full cutting structure, such as a tricone bit.
Therefore, a need has arisen for a drill string which may be used as a well casing or liner, which includes a drill bit on its lower end, and which, upon completion of drilling operations, may be retained within the wellbore without the need to retrieve the drill bit or the drill string. A need has also arisen for such a well casing or liner string that may be left in the wellbore along with a drill bit, and which does not require the use of expensive, unreliable or specialty equipment. Further, a need has arisen for such a well casing or liner string which may be cemented in place along with a drill bit having a full cutting structure.
The present invention, as exemplified by an embodiment disclosed herein, comprises a downhole drilling apparatus that is interconnectable in a casing or liner drill string and includes a drill bit connected thereto which, upon completion of drilling operations, may be retained within the wellbore without the need to retrieve the drill bit or the drill string. The apparatus allows the well casing or liner to be left in the wellbore along with the drill bit and does not require the use of expensive, unreliable or specialty equipment. The apparatus also allows for the well casing or liner to be cemented in place along with a drill bit having a full cutting structure.
The downhole drilling apparatus includes a housing that is interconnectable in a casing string. The housing has a window cut therein to allow a subsequent drill bit and pipe string to pass therethrough during a drill out operation. To facilitate the deflection of the drill bit and pipe string through the window, a whipstock is disposed within the housing. A filler material is also disposed within the housing between the whipstock and the window to prevent the flow of drilling fluids or cement through the window prior to the drill out. The filler and the whipstock have a central bore that permits the passage of fluids through the center of the downhole drilling apparatus. One or more valves may be disposed within the central bore to control the flow of fluids therethrough. The valves may be, for example, back pressure or float valves that allow one-way flow of fluids downwardly through the apparatus.
A drill bit having a full cutting structure, such as a tricone bit, may be operably coupled to the downhole drilling apparatus. The casing or liner string may be used to rotate the drill bit. Alternatively, a downhole motor may be coupled between the downhole drilling apparatus and the drill bit to facilitate rotation of the drill bit, without the need for rotating the casing string.
In another embodiment, a downhole drilling apparatus includes a housing having a window, an alignment member disposed within the housing and a back pressure valve assembly. The back pressure valve assembly includes a central bore that permits the passage of fluids therethrough. Once downhole, a whipstock may be run into the apparatus such that the whipstock operably engages the alignment member. The alignment member orients the whipstock within the housing relative to the window, so that the drill bit may subsequently be deflected through the window.
In operation, either embodiment of the downhole drilling apparatus may be interconnected in a casing or liner string having a drill bit disposed on its lower end. A first wellbore is drilled. Following the drilling of the first wellbore, the casing or liner string may be cemented within the wellbore. A pipe string having another drill bit on its lower end is passed through the casing or liner string, such that a drill out through the downhole drilling apparatus is performed to drill a second wellbore. The pipe string and drill bit that are used to create the second wellbore are deflected through the window in the housing of the downhole drilling apparatus by the whipstock disposed within the apparatus.
Thus, with the use of the downhole drilling apparatus, a casing or liner string including a drill bit having a full cutting structure may be used as a drill string to create a wellbore. The drill string may be cemented in place within the wellbore, and thereafter have a drill out performed therethrough to create an intersecting wellbore.
These and other features, advantages, benefits and objects of the present invention will become apparent to one of ordinary skill in the art upon careful consideration of the detailed description of representative embodiments of the invention hereinbelow and the accompanying drawings.